It is well known that photographic exposure of a photographic material is achieved by the combined effect of image intensity and the time this energy is allowed to act on the photographic material. Accurate control of the duration for which energy is allowed to act on the photographic material is therefore important. Shutter mechanisms are used to effect this control.
Shutter mechanisms can be categorised in accordance with their positioning within the camera, for example, front shutters, `between-lens` shutters and focal plane shutters. Most modern cameras employ a type of `between-lens` shutter.
In its simplest form, the `between-lens` shutter is a sector shutter comprising a circular metal plate with an elongated aperture. The metal plate is positioned perpendicular to the light path and makes one rotation when operated. Speed is controlled by a simple spring which is usually tensioned by a part of a release lever. Sector shutters are inexpensive and are often fitted to cheap cameras where inconsistencies which develop over a period of time and the limited speed range are not important.
The most commonly used type of photographic shutter is the bladed or diaphragm shutter. This type of shutter comprises three to five thin metal blades which open and shut across the light path. The blades are designed to minimise the time taken for opening and closing during the exposure cycle. Each shutter blade is pivoted at its outer edge and linked to a ring which is rotated to open the blades. Power for this rotation comes from a main driving spring which is tensioned prior to the shutter being activated. Diaphragm shutters are compact and offer a wide range of speeds.